Angular leaf spot

Xanthomonas fragariae

Angular leaf spot (ALS) is caused by a bacterium that primarily infects leaves. Although angular leaf spot was originally discovered in Minnesota in 1960, this disease is not a common problem today.

Identification

severe infection with angular leaf spot

U. Mazzucchi, Universitá di Bologna, Bugwood.org

water soaked appearance of lower leaf infected with angular leaf spot

D. Ferrin, Louisiana State University, Bugwood.org

dark sepals infected with angular leaf spot

G. Holmes, Valent Corporation, Bugwood.org

The first symptoms of angular leaf spot are water-soaked lesions on the underside of the leaf that are delineated by leaf veins creating an angular appearance. These lesions are best observed by picking a leaf and holding it up to the light, looking at the lower surface of the leaf. The spots will appear translucent with light behind them but will look dark green when the leaf is held in your hand. As the disease progresses the damage becomes visible on the upper surface of the leaf as reddish brown spots surrounded by a yellow halo. Heavily infected leaves may die. In warm wet weather, bacteria ooze out of infected tissue in slimy droplets that dry to a clear scaly film, similar to dried egg white. In severe infections, lesions can appear on the fruit caps that are identical to those on leaves. As the disease progresses the calyxes can also become dark brown and later dry up (ALS 6).

Important biology

Angular leaf spot bacteria are usually introduced to a berry patch on transplants that are infected but not showing symptoms. Under favorable weather conditions the bacteria ooze from leaf tissue and are dispersed by rain splash. The bacteria can then invade other plants through wounds or natural openings. This disease thrives in wet conditions with moderate daytime high temperatures (~ 68° F) and cold nights close to but above freezing (36-39° F). The angular leaf spot bacteria can overwinter in the crowns of live plants or in leaf debris.

Management

At present there are no strawberry cultivars resistant to angular leaf spot; however there is variation in susceptibility. The following cultivars have been shown to be highly susceptible, and should be avoided if angular leaf spot has been a problem in the past: Allstar, Annapolis, Cavendish, Honeoye, and Kent.

Purchase new plants from a reputable supplier and inspect all plants for symptoms of disease prior to planting. Accept only healthy symptom free plants.

Irrigate through drip irrigation or a soaker hose. If overhead sprinkling is your only option, water early in the morning on a sunny day so leaves dry quickly after irrigation.

Avoid working in an infected patch when the plants are wet as bacteria are easily spread on hands and tools at this time.

Use straw mulch to minimize water splashing.

Remove weeds to improve air circulation around plants.

Plant in rows or narrow beds, no wider than 12 - 18 inches to promote good air movement in and around plants. Patches grow with time as new runners are produced. Use renovation to maintain narrow beds.